This invention relates to an ink jet recording apparatus, and more particularly to improvements in the system detecting the state of ink droplets.
Ink jet recording apparatus of various types including an on-demand type and a charge control type are now available. In the on-demand type ink jet recording apparatus, ink droplets are produced or ejected from a nozzle toward and onto a recording medium when formation of recording dots is demanded. On the other hand, in the charge control type ink jet recording apparatus, ink droplets produced continuously from a nozzle are shielded by a gutter, and, when formation of recording dots is desired, the ink droplets are charged and deflected to be attached to a recording medium. In such ink jet recording apparatus, it is necessary to monitor the state of particulate ink or the state of charged ink for the purpose of improving the quality of records.
More precisely, the ink jet recording apparatus of the on-demand type is such that a pressurized ink container communicating with the nozzle is deformed by an electro-mechanical transducer element driven by a video information signal thereby causing ejection of ink droplets from the nozzle. Therefore, it is necessary to monitor as to whether or not the ink droplets are accurately produced in response to the application of the video information signal to the electro-mechanical transducer element. According to a monitoring method for monitoring accurate production of ink droplets, means for charging ink droplets ejected from the nozzle is provided so as to detect the quantity of charge of the charged ink droplets.
The ink jet recording apparatus of the charge control type is such that an electro-mechanical transducer element mounted on the nozzle ejecting ink is driven by a high-frequency power source to cause oscillation of ink so that the ink ejected from the nozzle is turned into droplets in synchronism with this oscillation. A charging electrode is disposed in the area where ink is turned into droplets, and a video information signal voltage is applied to the charging electrode to charge the ink droplets. An electrostatic field is established in the path of flight of the ink droplets to deflect the charged ink droplets thereby causing attachment of the charged ink droplets to a recording medium. Therefore, in the recording apparatus of this type, it is necessary to monitor as to whether or not the ink droplets are regularly produced and whether or not the charging is properly controlled. According to a monitoring method for monitoring regular production and charge control of the ink droplets, a signal voltage for checking the state of production of ink droplets is applied to the charging electrode with given timing, thereby detecting the quantity of charge of the charged ink droplets.
In such a state detecting system in which the quantity of charge of charged ink droplets is detected for the purpose of monitoring, for example, the state of particulate ink, an induction type sensor disposed in close proximity to the path of flight of ink or an impingement type sensor generating a detection signal indicative of the quantity of charge of charged ink droplets is commonly employed as means for detecting the quantity of charge of charged ink droplets. The detection signal current generated from each of these sensors is very weak or only about several nA. Therefore, in order that the particulate state of ink can be detected on the basis of such a very weak signal current, the detection signal must be amplified, and, for this purpose, an amplifier circuit having a high amplification factor is provided.
On the other hand, the ink jet recording apparatus includes, for example, an electrical circuit for driving the electro-mechanical transducer element, another electrical circuit for charging ink droplets, and a power source circuit for supplying power to these electrical circuits. A detection error may occur when electrical noise generated in any one of these electrical circuits is applied to the sensor and detection signal generating circuit. However, it is almost impossible from the aspect of practical use to prevent mixing of such electrical noise by means such as a shielding member.
U.S. Pat. No. 4,367,476 issued on Jan. 4, 1983 in the name of Syoji Sagae discloses one form of the ink jet recording apparatus of the charge control type in which the quantity of charge of charged ink droplets is detected to monitor the particulate state of ink.